Solid state imaging devices, such as charge coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors, have been used as a solid state imaging device in digital cameras. Usually containing millions of light receiving elements (for example, photodiodes) which correspond to each pixel of a single chip and convert light into a signal charge, the solid state imaging devices output an image signal representative of an optical image formed by a lens system. To capture high definition images, the solid state imaging devices are becoming to have more pixels in these days. When more pixels are incorporated in the digital camera, each light receiving element necessarily becomes smaller, and the sensitivity of the digital camera decreases. Therefore, micro-lenses are commonly introduced in front of the light receiving elements so as to enhance the focusing efficiency.
Some of the solid state imaging devices are equipped with light guides which direct incident light to the light receiving elements. The light guides are each disposed between the micro-lens and the light receiving element, and increase an amount of the light incident on the light receiving elements by reflecting the light toward the light receiving elements (see, for example, Japanese Patent Laid-open Publications No. 05-283661 and No. 2004-221532). The light guide of the publication No. 05-283661 is constituted of a light transmission film formed inside an opening of an insulating film and a reflecting surface over the opening. The light guide of the publication No. 2004-221532 is made of silicon nitride having a higher refractive index than a surrounding translucent medium, and guides the incident light to the light receiving element by total reflection.
These prior art light guides have a tapered shape with a large light entrance surface and a small light exit surface at both ends, so that they can take in the light from a wide area. Additionally, this small light exit surface serves to prevent the light out of the light guide from going to the transfer CCDs adjoining the light receiving element. This leads to prevent the image smear or such noises, which will otherwise be induced by the light spreading to enter the transfer CCDs.
When the tapered shape is incorporated, however, some drawbacks arise in the prior art light guides. For example, in the light guide of the publication No. 05-283661, the reflective surface slopes, and may reflect a part of the incident light toward the light entrance surface. In the light guide of the publication No. 2004-221532, a ray of light may occasionally strike a side surface of the light guide at an angle smaller than the critical angle, and passes through it. In other words, with the tapered shape, the prior art light guides would not direct the incident light to the light receiving elements efficiently.